The present invention relates to a photoreceptor, such as one for use in electrophotography, that has a light-sensitive layer composed of a carrier generating phase and a carrier transporting phase.
Conventional photoreceptors in electrophotography are typically inorganic in that they have light-sensitive layers primarily composed of inorganic photoconductive materials such as selenium, zinc oxide and cadmium. The inorganic photoreceptors do not necessarily perform satisfactorily in respect of sensitivity, heat stability, moisture resistance and durability. For example, selenium is easily deteriorated as a result of crystallization due to heat or in contact with extraneous material such as sebum from the fingers. Cadmium sulfide has problems in respect of moisture resistance and durability, and zinc oxide has low durability. Furthermore, selenium and cadmium sulfide do not have great latitude in manufacture and handling.
In recent years, the use of organic photoconductive materials in light-sensitive layers of photoreceptors has become the subject of intensive investigation. For example, Japanese Patent Publication No. 10496/1975 shows an organic photoreceptor having a light-sensitive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone. This photoreceptor is not necessarily satisfactory in respect to sensitivity and durability. In order to satisfy both requirements, attempts are being made to develop an organic photoreceptor having a light-sensitive layer wherein different materials perform two separate functions, one being carrier generation and the other being carrier transport. This photoreceptor, which may be called a function-separated type, has the advantage in that it permits selecting most suitable functional materials from a wide range of candidates and that therefore a photoreceptor having desired characteristics can be obtained with relative ease.
Many materials have been proposed as effective carrier generating materials for use in photoreceptors of the function-separated type. An exemplary inorganic material is amorphous selenium as shown in Japanese Patent Publication No. 16198/1968. While amorphous selenium is combined with an organic carrier transport material, a carrier generation layer composed of amorphous selenium is easily deteriorated as a result of crystallization due to heat or in contact with extraneous matter.
Several photoreceptors that use organic dyes or pigments as carrier generation materials have also been proposed. For example, photoreceptors having light-sensitive layers containing bisazo compounds are shown in Japanese Patent Application (OPI) Nos. 37543/1972, 22834/1980, 79632/1979 and 116040/1981 (the symbol OPI as used herein means an unexamined published Japanese patent application). However, none of the photoreceptors shown in these patents satisfy a wide spectrum of requirements in electrophotographic process: they are not completely satisfactory in respect of sensitivity, residual potential or stability after cyclic operation, and in addition, the latitude for selection of carrier transport materials that can be used in these photoreceptors is limited. Known bisazo compounds exhibit a fairly good sensitivity in the short- to medium wavelength range, but they have no sensitivity in the long wavelength range. If a tungsten lamp is used as a light source, the long wavelength component of the light is simply wasted, and a semiconductor laser or other sources that issue light in long-wavelength region cannot be used with such bisazo compounds. Because of this limit on the usable wavelength range, photoreceptors having light-sensitive layers which contain bisazo compounds cannot be used in a wide scope of applications.
A further problem with photoreceptors is that a carrier transport material which is effective for use in combination with a certain carrier generation material may prove ineffective with another carrier generation material; on the other hand, a carrier generation material which is effective for use in combination with a certain carrier transport material may prove useless when combined with another carrier transport material. If an improper combination of carrier generation and transport materials is used, the resulting photoreceptor has a low sensitivity; what is more, the low discharge efficiency in a weak electric field causes a high "residual potential", and in an extreme case, the residual potential increases after each cycle of operation, and the photoreceptor eventually becomes no longer usable.
There is no general rule that can be relied upon for selected optimum combinations of a material that is to be incorporated in a carrier generation phase and a material that should be present in a carrier transport phase. It is therefore necessary to determine advantageous combinations by screening a wide scope of materials on a trial and error basis.
Known photoreceptors using organic photoconductive materials are usually designed for negative charging, because high hole mobility provides a favorable condition for high photosensitivity. However, researchers have shown that negatively charged photoreceptors cause two problems. One is the formation of ozone that is deleterious to the operation environment. The other problem is the need for using a toner of positive polarity in developing the negatively charged photoreceptor; in view of the triboelectric series with respect to the particles of a ferromagnetic carrier, the toner of positive polarity is difficult to manufacture.
The positive charging of a photoreceptor using an organic photoconductive material has been proposed by several researchers. According to one proposal, a carrier generation layer is overlaid with a carrier transport layer made of a material having a high electron transport efficiency, but this idea is not acceptable since trinitrofluorenone incorporated in the carrier transport layer has carcinogenicity. Another possible arrangement would consist of a carrier transport layer that has a high hole transport efficiency and which is overlaid with a carrier generation layer. However, this is also not practical since the carrier generation layer located on top of the photoreceptor makes it unsuitable for cyclic operation.
U.S. Pat. No. 3,615,414 shows a photoreceptor for positive charging, wherein a thiapyrylium salt (carrier generating material) is so incorporated that if forms a eutectic complex with a polycarbonate (binder resin). But this product is defective in that it experiences an appreciable degree of "memory effect" and is highly likely to cause ghost. U.S. Pat. No. 3,357,989 shows a photoreceptor using phthalocyanine. However, phthalocyanine requires a strict control over its crystal type since its characteristics are highly dependent on the crystal type. Furthermore, phthalocyanine has an insufficient sensitivity to light in the short wavelength range and experiences a significant degree of memory effect". The photoreceptor of U.S. Pat. No. 3,357,989 is not suitable for use with a copier of the type that operates on a light source in the visible wavelength range.
These are the reasons why photoreceptors using organic photoconductive materials have been exclusively designed for use in the negatively charged state.
One serious problem with the conventional photoreceptors in electrophotography is that the light-sensitive layer is electrically fatigued so much as a result of repeated electrophotographic processing that the receptors have a very short service life. When one cycle of electrophotographic processing has been completed, any residual charges on the light-sensitive layer must be cleared before another cycle is started. However, the light-sensitive layer used in conventional photoreceptors has such a low discharge rate in the discharging step that even a large quantity of flooding light is unable to erase the electrostatic image completely, and an appreciably high residual potential is left. This residual potential increases in a cumulative manner as a result of repeated electrophotographic processing, and only after a few cycles of copying operation, the residual potential exceeds the maximum permissible level, making the photoreceptor no longer usable.
Even in such a case, certain types of photoreceptors can be put into a re-usable condition, but this requires a considerably long shutdown period or an appropriate heat treatment. Furthermore, the residual potential cannot be reduced to a sufficiently low level, and the photoreceptor will soon become inoperable again.
A photoreceptor combining an electron donating, carrier transport material with a carrier generation material is known. A method has been proposed to incorporate a very small amount of a Lewis acid within the carrier transport layer for the purposes of preventing the accumulation of residual potential and improving the ability of cyclic operation. This method is effective only with a photoreceptor using a particular kind of electron donating carrier transport material, and with photoreceptors using many other kinds of electron donating carrier materials, the method is unable to achieve sufficient prevention of residual potential buildup. This is particularly true with a photoreceptor using a polyarylalkane aromatic amino compound as a carrier transport material since the device is greatly deteriorated due to UV radiation and other factors.
Amine derivatives have been proposed for use as a charge transport material in the light-sensitive layer (Japanese Patent Application No. 8044/1981). While amine derivatives have various advantages, they also have several problems to be solved. If an amine derivative is singly used as a charge transport material (CTM), the discharge curve has a relatively long tail and the sensitivity of the photoreceptor is likely to become low in a weak electric field. Furthermore, the photoreceptor is instable against UV radiation and has the great change of being deteriorated during maintenance under a fluorescent lamp or daylight.